Relationship of a 5-HT transporter functional polymorphism to 5-HT1A receptor binding in healthy women

Depression and Obesity: Integrating the Role of Stress, Neuroendocrine Dysfunction and Inflammatory Pathways

Literature on depression and obesity describes the relevance of the hypothalamic pituitary adrenal axis dysfunction, sympathetic nervous system (SNS) activation, and inflammatory processes as well as the interaction of genetic and environmental factors. Recent investigation in obesity highlights the involvement of several regulation systems, particularly in white adipose tissue. The hypothalamic pituitary adrenal axis, gonadal, growth hormone, leptin, sympathetic nervous system and adrenergic, dopaminergic, and serotoninergic central pathways, all seem interconnected and involved in obesity. From another perspective, the role of psychosocial chronic stressors, determining poor mental and physical health, is well documented. Empirical data can support biologically conceivable theories describing how perceptions of the external social environment are transduced into cellular inflammation and depression. Although in neurobiological models of depression, stress responses are associated with neuroendocrine and neuro-inflammatory processes, concerning similar pathways to those described in obesity, an integrating model is still lacking. The aim of this mini-review is to offer a reflexion on the interplay between the neuroendocrine dysfunctions related to chronic stress and the nature of the shared biologic mechanisms in the pathophysiology of both clinical entities, depression and obesity. We highlight dysfunctional answers of mind body systems that are usually activated to promote regulation and adaptation. Stress response, as a mediator between different level phenomena, may undertake the role of a plausible link between psychological and biological determinants of disease. Depression and obesity are major public health issues, urging for new insights and novel interventions and this discussion points to the need of a more in-depth approach.

Conservation of 5-HT1A receptor-mediated autoinhibition of serotonin (5-HT) neurons in mice with altered 5-HT homeostasis

Firing activity of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) is controlled by inhibitory somatodendritic 5-HT_1A autoreceptors. This autoinhibitory mechanism is implicated in the etiology of disorders of emotion regulation, such as anxiety disorders and depression, as well as in the mechanism of antidepressant action. Here, we investigated how persistent alterations in brain 5-HT availability affect autoinhibition in two genetically modified mouse models lacking critical mediators of serotonergic transmission: 5-HT transporter knockout (Sert-/-) and tryptophan hydroxylase-2 knockout (Tph2-/-) mice. The degree of autoinhibition was assessed by loose-seal cell-attached recording in DRN slices. First, application of the 5-HT_1A-selective agonist R(+)-8-hydroxy-2-(di-n-propylamino)tetralin showed mild sensitization and marked desensitization of 5-HT_1A receptors in Tph2-/- mice and Sert-/- mice, respectively. While 5-HT neurons from Tph2-/- mice did not display autoinhibition in response to L-tryptophan, autoinhibition of these neurons was unaltered in Sert-/- mice despite marked desensitization of their 5-HT_1A autoreceptors. When the Tph2-dependent 5-HT synthesis step was bypassed by application of 5-hydroxy-L-tryptophan (5-HTP), neurons from both Tph2-/- and Sert-/- mice decreased their firing rates at significantly lower concentrations of 5-HTP compared to wildtype controls. Our findings demonstrate that, as opposed to the prevalent view, sensitivity of somatodendritic 5-HT_1A receptors does not predict the magnitude of 5-HT neuron autoinhibition. Changes in 5-HT_1A receptor sensitivity may rather be seen as an adaptive mechanism to keep autoinhibition functioning in response to extremely altered levels of extracellular 5-HT resulting from targeted inactivation of mediators of serotonergic signaling.

Depression and epilepsy: epidemiologic and neurobiologic perspectives that may explain their high comorbid occurrence

Depression is the most frequent psychiatric comorbidity in people with epilepsy (PWE) with lifetime prevalence rates ranging between 30 and 35%. Multifactorial variables play a pathogenic role in the high comorbid occurrence of these two disorders. These variables were critically examined during an international symposium held in Chicago in September 2010, the results of which are presented in two companion manuscripts. The first manuscript summarizes new epidemiologic data highlighting the bidirectional relation between depression and epilepsy and related methodological issues in studying this relationship. An examination of the neurobiologic aspects of primary mood disorders, mood disorders in PWE and pathogenic mechanisms of epilepsy derived from studies in animal models and humans is allowing a better understanding of the complex relation between the two conditions. In the first manuscript, we review data from animal models of epilepsy in which equivalent symptoms of depression and anxiety disorders develop and, conversely, animal models of depression in which the kindling process is facilitated. Data from structural and functional neuroimaging studies in humans provide a further understanding of potential common pathogenic mechanisms operant in depression and epilepsy that may explain their high comorbidity. The negative impact of depression on the control of seizure disorders has been documented in various studies. In this manuscript, these data are reviewed and potential mechanisms explaining this phenomenon are proposed.

The genetic blueprint of major depressive disorder: contributions of imaging genetics studies

OBJECTIVES

To review neuroimaging intermediate phenotypes of MDD and their relation to genetic risk variants.

METHODS

A systematic literature search of peer-reviewed English language articels using PubMed ( www.pubmed.org ) was performed.

RESULTS

Comprehensive evidence on the influence of serotonergic genes (SLC6A4, HTR1A, MAOA, TPH2) and BDNF on the following neural intermediate phenotypes is displayed: amygdala reactivity, coupling of amygdala-anterior cingulate cortex (ACC) activity, ACC volume, hippocampal volume and serotonin receptor 1A (5-HT1A) binding potential (BP).

CONCLUSIONS

Intermediate phenotypes may bridge the gap between genotype and phenotype by reducing the impreciseness of psychiatric phenotypes and yield more insights into the underlying biology.

Pharmacogenetics of antidepressive treatment

The search of the genetic predictors of response to antidepressants is a rapidly expanding field. A large number of clinical studies are reporting partly inconsistent results. Emerging new results focus on new candidate single nucleotide polymorphisms--particularly in the 5HT2a-receptor gene and the gene coding for the co-chaperone FKBP5. The impact of the 5HTTLPR polymorphism on therapeutic outcome and side effects under treatment with SSRIs has to be viewed in a more complex manner than previously proposed. All replicable genetic associations display only a very modest effect. Despite of enormous research efforts, currently pharmacogenetics of therapeutic effects and of side effects of antidepressants are unable to guide decisions on the selection of the most beneficial drug for an individual patient.

5-HT1A gene promoter polymorphism and [18F]MPPF binding potential in healthy subjects: a PET study

Background

Previous Positron Emission Tomography (PET) studies of 5-HT_1A receptors have shown an influence of several genetic factors, including the triallelic serotonin transporter gene-linked polymorphic region on the binding potential (BP_ND) of these receptors. The aim of our study was to investigate the relationship between a 5-HT_1A promoter polymorphism and the binding potential of another selective 5-HT_1A receptor antagonist, [¹⁸F]MPPF, in healthy subjects.

Methods

Thirty-five volunteers, including 23 women, underwent an [¹⁸F]MPPF scan and were genotyped for both the C(-1019)G 5-HT_1A promoter polymorphism and the triallelic serotonin transporter gene-linked polymorphic region. We used a simplified reference tissue model to generate parametric images of BP_ND. Whole brain Statistical Parametric Mapping and raphe nuclei region of interest analyses were performed to look for an association of [¹⁸F]MPPF BP_ND with the C(-1019)G 5-HT_1A promoter polymorphism.

Results

Among the 35 subjects, 5-HT_1A promoter genotypes occurred with the following frequencies: three G/G, twenty-one G/C, and eleven C/C. No difference of [¹⁸F]MPPF BP_ND between groups was observed, except for two women who were homozygote carriers for the G allele and showed greater binding potential compared to other age-matched women over the frontal and temporal neocortex. However, the biological relevance of this result remains uncertain due to the very small number of subjects with a G/G genotype. These findings were not modified by excluding individuals carrying the S/S genotype of the serotonin transporter gene-linked polymorphic region.

Conclusions

We failed to observe an association between the C(-1019)G 5-HT_1A promoter polymorphism and [¹⁸F]MPPF binding in healthy subjects. However our data suggest that the small number of women homozygote for the G allele might have greater [¹⁸F]MPPF BP_ND relative to other individuals. This finding should be confirmed in a larger sample.

Genetic sensitivity to the environment: the case of the serotonin transporter gene and its implications for studying complex diseases and traits

Evidence of marked variability in response among people exposed to the same environmental risk implies that individual differences in genetic susceptibility might be at work. The study of such Gene-by-Environment (GxE) interactions has gained momentum. In this article, the authors review research about one of the most extensive areas of inquiry: variation in the promoter region of the serotonin transporter gene (SLC6A4; also known as 5-HTT) and its contribution to stress sensitivity. Research in this area has both advanced basic science and generated broader lessons for studying complex diseases and traits. The authors evaluate four lines of evidence about the 5-HTT stress-sensitivity hypothesis: 1) observational studies about the serotonin transporter linked polymorphic region (5-HTTLPR), stress sensitivity, and depression in humans; 2) experimental neuroscience studies about the 5-HTTLPR and biological phenotypes relevant to the human stress response; 3) studies of 5-HTT variation and stress sensitivity in nonhuman primates; and 4) studies of stress sensitivity and genetically engineered 5-HTT mutations in rodents. The authors then dispel some misconceptions and offer recommendations for GxE research. The authors discuss how GxE interaction hypotheses can be tested with large and small samples, how GxE research can be carried out before as well as after replicated gene discovery, the uses of GxE research as a tool for gene discovery, the importance of construct validation in evaluating GxE research, and the contribution of GxE research to the public understanding of genetic science.

Imaging the effects of genetic polymorphisms on radioligand binding in the living human brain: A review on genetic neuroreceptor imaging of monoaminergic systems in psychiatry

Imaging genetics is a research field that describes the impact of genetic risk variants on brain structure and function. While magnetic resonance based imaging techniques are able to provide complex information on a system level, positron emission tomography (PET) and single photon emission computer tomography (SPECT) allow for determination of distribution and density of single receptor molecules in the human brain. Major psychiatric disorders are highly heritable, and have been associated with a dysregulation in brain dopamine and serotonin systems. Understanding the role of genetic polymorphisms within these neurotransmitter systems on brain phenotype is essential. This review tries to cover the literature on the impact of gene variants implicated in psychiatric disorders on serotonin, dopamine, and MAO-A radioligand binding in living humans. The majority of PET and SPECT studies investigated the role of polymorphisms within genes coding for the serotonin and dopamine transporters, the serotonin 1A receptor, and the dopamine D2 receptor on G protein coupled receptors or transporter proteins critically involved in serotonin or dopamine neurotransmission. Other studies investigated the impact of variants in genes for monoamine oxidase-A (MAO-A) or brain derived neurotrophic factor on monoamine transporters, receptors, or MAO-A activity. Two main findings in healthy subjects emerge from the current literature: one is an increased binding of the selective ligand [(11)C]DASB to serotonin transporters in subjects homozygous for the triallelic 5-HTTLPR LA allele. The other one is decreased binding of the radioligand [(11)C]raclopride to dopamine D2 receptors in D2 Taq1 A1 allele carriers. Other findings reported are highly interesting but require independent replication.

Serotonin transporter genotype is associated with cognitive performance but not regional 5-HT1A receptor binding in humans

The human serotonin transporter (5-HTT) gene is one of the most extensively studied in psychiatry. A functional polymorphism in the promoter region of the 5-HTT gene (5-HTTLPR) has been associated with several psychiatric disorders as well as anxiety-related personality traits. In search of a mechanistic understanding of the functional implications of 5-HTTLPR, the influence of this polymorphism on regional 5-HT1A receptor density has previously been examined in two positron emission tomography (PET) studies in humans, yielding, however, contradictory results. In the present study, 54 control subjects were examined with [11C]WAY 100635 PET and a battery of cognitive tests. Regional binding potential (BP) of [11C]WAY 100635 to 5-HT1A receptor was calculated for the dorsal raphe nuclei, the hippocampus, the anterior cingulate, the insula, the temporal cortex and the frontal cortex. The influence of 5-HTTLPR genotype on regional 5-HT1A BP and cognitive performance was investigated. No differences in 5-HT1A receptor density between carriers and non-carriers of the S allele were found. Thus, we could not replicate any of the previously reported associations between 5-HTTLPR and 5-HT1A density. There was, however, a highly significant association between 5-HTTLPR genotype and performance in Wisconsin Card Sorting Test; carriers of the S allele had a superior performance compared to the LL carriers. These observations suggest that functional implications of the 5-HTTLPR polymorphism are not likely to be mediated by differences in 5-HT1A expression levels and that other biomarkers must be considered for future investigations at phenotype level.

Association between triallelic polymorphism of the serotonin transporter and [18F]MPPF binding potential at 5-HT1A receptors in healthy subjects

Previous [(11)C]WAY100-635 PET studies have demonstrated that the short (S) and long (L) alleles of the serotonin transporter gene-linked polymorphic region (5-HTTLPR) were associated with distinct patterns of 5-HT(1A) receptor distribution in human. However, these studies reported discordant findings and did not take into account the recent description of two functional variants of the L allele (L(A)/L(G)). To further explore this issue, we investigated the triallelic functional polymorphism of the 5-HTTLPR in 38 healthy volunteers who underwent a [(18)F]MPPF PET study of 5-HT1A receptors. We used a simplified reference tissue model to generate parametric images of [(18)F]MPPF binding potential (BP(ND)), and compared these data among the different genotypes using statistical parametric mapping and region of interest of the raphe nuclei. Homozygote carriers of the S allele demonstrated greater [(18)F]MPPF BP(ND) than carriers of the L(A) allele, but this association was only found in women. Differences in [(18)F]MPPF BP(ND) between women with and without L(A) allele were observed over large clusters encompassing the right and left temporal lobes, cingulate and perisylvian regions, as well as the right precuneus and frontal dorso-lateral cortex, and the left orbitofrontal cortex. In contrast, no difference was found between groups in the raphe nuclei. The greater [(18)F]MPPF BP(ND) observed in women homozygote carriers of the S allele could either reflect a greater 5-HT1A receptor density or a lower extracellular concentration of 5-HT. Our data suggest that any future PET studies of 5-HT1A receptors should incorporate the 5-HTTLPR polymorphism status of the population studied.

Risk for depression during interferon-alpha treatment is affected by the serotonin transporter polymorphism

BACKGROUND

Major depressive disorder (MDD) occurs in a subset of patients receiving interferon-alpha treatment, although many are resilient to this side effect. Genetic differences in the serotonin reuptake transporter promoter (5-HTTLPR) may interact with the inflammatory system and influence depression risk.

METHODS

A cohort of 71 nondepressed hepatitis C patients about to receive interferon-alpha was prospectively followed, employing a diagnostic structured clinical interview (Structured Clinical Interview for DSM-IV Axis I Disorders [SCID-I]) and self-report questionnaires. Patients were genotyped for the 5-HTTLPR (L(G), L(A), and S) and the variable number of tandem repeats (VNTR) polymorphism in the second intron. Kaplan-Meier analyses were used to compare major depression incidence. Genotype effects on sleep quality (Pittsburgh Sleep Quality Index) and Beck Depression Inventory (BDI) were assessed using mixed-effect repeated-measure analyses.

RESULTS

The L(A) allele was associated with a decreased rate of developing MDD (Mantel-Cox log rank test p < .05) with the L(A)/L(A) genotype being the most resilient. This genotype was also associated with better sleep quality [F(61.2,2) = 3.3, p < .05]. The ability of baseline sleep quality to predict depression incidence disappeared when also including genotype in the model. Conversely, the relationship of neuroticism with depression incidence (B = .07, SE = .02, p < .005) was not mitigated when including genotype.

CONCLUSIONS

Using a prospective design, 5-HTTLPR is associated with MDD incidence during interferon-alpha treatment. Preliminary evidence that this effect could be mediated by effects on sleep quality was observed. These findings provide support for a possible interaction between inflammatory cytokine (interferon-alpha) exposure and 5-HTTLPR variability in MDD.

Transcriptional regulation at a HTR1A polymorphism associated with mental illness

The serotonin-1A (5-HT1A) receptor serves as a hub to regulate the activity and actions of the serotonin system, and is expressed both as a presynaptic autoreceptor on raphe neurons, and as a major postsynaptic receptor in hippocampal, cortical, and hypothalamic regions involved in mood, emotion and stress response. As such, the level of expression of 5-HT1A receptors is implicated in the development of anxiety and depression phenotypes. This review focuses on the C(-1019)G (rs6295) promoter polymorphism of the 5-HT1A receptor gene (HTR1A) and its effect on the activity of transcription factors that recognize the C-allele, including Deaf-1, Hes1 and Hes5; its effects on 5-HT1A receptor expression in pre- and postsynaptic areas; as well as its implication in early postnatal development and adult neurogenesis in the hippocampus and cortex. Although several studies have now replicated the association of the G-allele with depression, panic disorder, neuroticism, and reduced response to antidepressant or antipsychotic treatment, ethnic, disease and genetic heterogeneity among subjects in different studies may obscure such associations. Gene-gene interaction studies suggest that the 5-HT1A receptor G(-1019) allele is a risk allele which could be used as a marker for depression and related mood disorders. Finally, association of the G(-1019) allele with increased raphe 5-HT1A binding potential, increased amygdala reactivity to emotional stimuli, and reduced amygdala volume, particularly in disease states, suggests a functional role for the C(-1019)G site in 5-HT1A receptor dys-regulation and predisposition to mental illness.

Identification of neurogenetic pathways of risk for psychopathology

Imaging genetics has been a highly effective and increasingly applied strategy for identifying the impact of genetic polymorphisms on individual differences in neural circuitry supporting complex behaviors. The application of imaging genetics towards further elucidating neural circuitry associated with the pathophysiology of psychiatric illness is of particular interest given its potential to guide the development and improvement of current therapeutic methods. The identification of genetic variants that contribute to or predict the disruption of specific neural pathways associated with psychopathology may also serve as useful markers of risk demarcating individuals with elevated susceptibility for psychiatric illness and affording early or even preemptive treatment strategies. In the continued development of this technique, recent multimodal neuroimaging strategies and studies examining the effects of multiple genes in concert within large subject populations have shown promise in the development of a more complete understanding of the interrelationships between genes, brain function, behavior and associated risk for psychopathology.

Genetic variation in cortico-amygdala serotonin function and risk for stress-related disease

The serotonin system is strongly implicated in the pathophysiology and therapeutic alleviation of stress-related disorders such as anxiety and depression. Serotonergic modulation of the acute response to stress and the adaptation to chronic stress is mediated by a myriad of molecules controlling serotonin neuron development (Pet-1), synthesis (tryptophan hydroxylase 1 and 2 isozymes), packaging (vesicular monoamine transporter 2), actions at presynaptic and postsynaptic receptors (5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, 5-HT3A, 5-HT4, 5-HT5A, 5-HT6, 5-HT7), reuptake (serotonin transporter), and degradation (monoamine oxidase A). A growing body of evidence from preclinical rodents models, and especially genetically modified mice and inbred mouse strains, has provided significant insight into how genetic variation in these molecules can affect the development and function of a key neural circuit between the dorsal raphe nucleus, medial prefrontal cortex and amygdala. By extension, such variation is hypothesized to have a major influence on individual differences in the stress response and risk for stress-related disease in humans. The current article provides an update on this rapidly evolving field of research.

Neuroimaging: technologies at the interface of genes, brain, and behavior

Neuroimaging technologies provide a powerful approach to exploring the genetic basis of individual differences in complex behaviors and vulnerability to neuropsychiatric illness. Functional MRI studies have established important physiologic links between genetic polymorphisms and robust differences in information processing within distinct brain regions and circuits that have been linked to the manifestation of various disease. Neuroimaging technologies represent a critical tool in efforts to understand the neurobiology of normal and pathologic behavioral states. Research capitalizing on neuroimagingbased integration will contribute to the identification of predictive markers and biologic pathways for neuropsychiatric disease vulnerability and the generation of novel targets for therapeutic intervention.

Anxiety and cognitive effects of quercetin liposomes in rats

Quercetin, an effective flavonol used as an antioxidant, was investigated for its anxiolytic and cognitive activities in male Wistar rats. Oral quercetin (300 mg/kg body weight/day) was compared with oral and intranasal quercetin liposomes (20 microg/day). Quercetin liposomes, in a mixture of egg phosphatidylcholine, cholesterol, and quercetin (2:1:1) and dispersed in 50% polyethylene glycol in water, were approximately 200 nm in mean particle diameter and negative surface charge with a range of encapsulation efficiency of 60% to 80%. Anxiolytic and cognitive-enhancing effects of quercetin, conventional and liposomal, were subjected to elevated plus maze and Morris water maze tests, respectively. Both conventional and quercetin liposomes showed anxiolytic and cognitive-enhancing effects. A lower dose and a faster rate were observed with intranasal quercetin liposomes when compared with oral quercetin, conventional and liposomal. The intranasal quercetin liposomes are effective in the delivery of quercetin to the central nervous system.

Sex differences in the serotonin 1A receptor and serotonin transporter binding in the human brain measured by PET

Women and men differ in serotonin associated psychiatric conditions, such as depression, anxiety and suicide. Despite this, very few studies focus on sex differences in the serotonin system. Of the biomarkers in the serotonin system, serotonin(1A) (5-HT(1A)) receptor is implicated in depression, and anxiety and serotonin transporter (5-HTT) is a target for selective serotonin reuptake inhibitors, psychotropic drugs used in the treatment of these disorders. The objective of the present study was to study sex related differences in the 5-HT(1A) receptor and 5-HTT binding potentials (BP(ND)s) in healthy humans, in vivo. Positron emission tomography and selective radioligands [(11)C]WAY100635 and [(11)C]MADAM were used to evaluate binding potentials for 5-HT(1A) receptors (14 women and 14 men) and 5-HTT (8 women and 10 men). The binding potentials were estimated both on the level of anatomical regions and voxel wise, derived by the simplified reference tissue model and wavelet/Logan plot parametric image techniques respectively. Compared to men, women had significantly higher 5-HT(1A) receptor and lower 5-HTT binding potentials in a wide array of cortical and subcortical brain regions. In women, there was a positive correlation between 5-HT(1A) receptor and 5-HTT binding potentials for the region of hippocampus. Sex differences in 5-HT(1A) receptor and 5-HTT BP(ND) may reflect biological distinctions in the serotonin system contributing to sex differences in the prevalence of psychiatric disorders such as depression and anxiety. The result of the present study may help in understanding sex differences in drug treatment responses to drugs affecting the serotonin system.

The serotonin transporter 5-HTTLPR polymorphism and treatment response to nicotine patch: follow-up of a randomized controlled trial

In this follow-up of a randomized placebo-controlled clinical trial of nicotine replacement transdermal patch for smoking cessation, 741 smokers of European ancestry who were randomized to receive active patch or placebo patch were genotyped for the serotonin transporter gene-linked polymorphic region. The study setting was a primary care research network in Oxfordshire, United Kingdom. The primary outcome measures were biochemically verified sustained abstinence from cigarette smoking at end of treatment and 24-week follow-up. The main effect of genotype was not associated with sustained abstinence from smoking at either end of treatment (SL: p=.33; SS: p=.81) or 24-week follow-up (SL: p=.05; SS: p=.21), and we found no evidence for a genotypextreatment interaction effect. In summary, despite the theoretically important contribution of serotonin neurotransmission to smoking cessation, the serotonin transporter gene was not associated with treatment response to nicotine patch for smoking cessation in this primary care-based trial.

Genetics of emotional regulation: the role of the serotonin transporter in neural function

Identifying biological mechanisms through which genes lead to individual differences in emotional behavior is paramount to our understanding of how such differences confer risk for neuropsychiatric illness. The emergence of techniques such as in vivo imaging of brain function in humans and genetic engineering in rodents has provided important new insights into the impact of serotonin (5-HT), a key modulator of emotional behavior, on neural systems subserving anxiety and depression. A major finding has been the discovery of genetic variation in a crucial regulatory molecule within the 5-HT system, the 5HT transporter (5-HTT), and its influence on emotional traits. The study of the 5-HTT provides a new foundation for understanding the neurobiological and genetic basis of emotional regulation and affective illness.